[PM]`PredictionMarkets` can use any approved token as a base currency

[lsaether]

Currently, the PredictionMarkets pallet expects that only the native currency (ZTG) is being used as the base currency for all prediction markets to settle in. However, stablecoin like kUSD will also be a popular choice when creating markets, and should be able to be used as a base currency as well.

• There should be a list of approved tokens that can be used as base currency for prediction markets. The list of approved tokens can be changed by governance or the root origin.
• We should add USD as a token type in the Asset enum, which currently is just stubbed.
• PredictionMarkets internal logic should be changed from assuming that all markets are using ZTG as the base currency, to using any of the approved assets.